In recent years, a rectilinear motion-type press (hereinafter referred to as a servo press) in which a slide is rectilinearly driven by an electric servo motor such as an AC servo motor or the like is used in many fields. The reason for the above is that compared with conventional mechanical presses, the servo press has advantages of being able to operate with freer slide motion at higher speed and to easily control pressure, thus enabling a higher degree of forming and improving productivity. Further, as compared with oil hydraulic presses, the servo press has advantages of providing lower noise, lower vibration, and higher energy saving, being clean without oil leakage, and being able to start and stop a slide extremely in a short time, thus improving working environment, productivity, and the like.
Known as a controller for such a servo press is a servo press controller disclosed in Japanese Patent Publication No. 3-33439 which shows a controller which operates predetermined servo calculation processing with one computer unit (hereinafter called CPU) as a core, outputs the servo command to a servo pack (a so-called servo amplifier) to control the position and speed of a servo motor, and rectilinearly drives a ram (a slide) with the stroke thereof being within a previously set range.
In the aforesaid conventional servo press controller, however, there occur disadvantages regarding safety described below.
1) When a CPU chip malfunctions, for example, due to thermal runaway, abnormalities in power supply voltage, or electrical noise, the maximum value of speed command or position command is sometimes outputted. Thereby, there is the possibility that so-called servo runway occurs, whereby the ram abruptly operates at high speed in an unexpected direction.
2) Further, when a ram position sensor breaks down, some electrical noise is intermingled with a signal line of the position sensor, or a contact failure in a input connector of position signals and the like occurs, there is the possibility that the position control of the ram becomes impossible, thereby producing servo runaway.
3) Furthermore, when the control software of the CPU malfunctions due to some program error and the like and so-called software runaway occurs, there is also the possibility that servo runaway occurs.